In a conventional FT (Fourier Transform) optical spectrometer, a Michelson interferometer is used, where a broadband light source is divided into two equal-intensity beams with one of the beams traversing a fixed-distance path and the other traversing a variable-distance path. The beams are then recombined and focused onto a detector. There are some major disadvantages of conventional FT spectrometers, such as the requirement of large dynamic range amplifiers and large-bit A/D boards, the alignment and mechanical reliability problem caused by a moving mirror, and the inability to directly ratio the sample and background signals simultaneously.
Accordingly, it is an object of U.S. Pat. No. 6,031,609, “Fourier transform spectrometer using a multi-element liquid crystal display”, disclosed by Funk, et al. to provide an apparatus and method for performing Fourier transform spectroscopy using fixed dispersive optical elements and a polychromatic light source with no bulk optical moving parts. Another object of that reference is to provide a Fourier transform spectrometer having increased S/N ratios over scanning dispersive instruments and gain substantial freedom from 1/f noise. However, Funk's invention is limited due to the transparent wavelength limitation of liquid crystal, especially in the longer wavelength infrared region. In addition, there is a 3 dB loss due to the requirement of polarized light input.
An independent area of endeavor concerns MEMS (micro-electro-mechanical systems) technology. Traditionally, MEMS devices are derivatives of moving mirrors and operate as tiltable reflective surfaces. These are true micro-machines that incorporate actual mechanical components, such as mirrors mounted on some form of a mechanical bearing device. The source light is reflected into different directions as the mirror sweeps across an arc. In many tilting mirror designs, the MEMS device is etched out of a silicon substrate, with the control surface coated with a reflective material such as gold or aluminum, leaving a mirror on a bearing surface. In operation, this type of device will “sweep” light at constant amplitude from the source to the destination aperture, such as an optical fiber or exit pupil. In other words, the light amplitude is constant while the output angle is variable.
The applications of these tiltable mirror or deformable mirror based MEMS devices in the spectrometer field have been disclosed by Stafford in U.S. Pat. No. 5,504,575, “SLM Spectrometer”, Messerschmidt in U.S. Pat. No. 5,828,066, “Multisource Infrared Spectrometer”, Fateley in U.S. Pat. No. 6,128,078, “Radiation Filter, Spectrometer and Imager Using a Micro-Mirror Array”, and Polynkin et al. in U.S. Pat. No. 6,753,960 B1, “Optical Spectral Power Monitors Employing Frequency-Division-Multiplexing Detection Schemes”. In the patents by Stafford, Fateley and Polynkin, a MEMS mirror device in combination with a dispersive element is placed between a sample and a photo detector to measure the emission spectrum of the sample. The MEMS mirror functions as either a filter to select certain wavelength component from the emission spectrum or a modulator to encode each wavelength component with different modulation frequency. The drawback of these approaches is that they do not provide a direct way to ratio the spectrum of the sample with the spectrum of the light source, i.e. the background signal, which is very important for absorption or reflection spectrum measurement.
In the patent by Messerschmidt, a deformable MEMS mirror array is employed in combination with a diffraction grating to form a monochrometer to select the desired illumination wavelength from a light source. The MEMS device is placed between the light source and the grating element and functions as an apparatus to spatially modulate the waveform of the light source by varying the curvature of the micro mirror. The performance of this spectroscopic system is not ideal because the waveform modulation provided by the micro mirror is not very accurate. What is more, the disclosed optical system does not provide background signal calibration. None of the patents discussed above teach or suggest the application of the MEMS based spectrometer in the mid or far infrared wavelength region (with wavelength of >2.5 μm).
A different kind of MEMS device, known as diffractive MEMS (D-MEMS), utilizes the wave aspect of light, i.e., interference and diffraction. The basic technology, originally referred to as deformable grating modulators, was pioneered at Stanford University in the early 1990s (O. Solgaard, F. S. A. Sandejas, and D. M. Bloom, “Deformable grating optical modulator,” Opt. Lett. 17, 9, pp. 688-690, 1992). The design of D-MEMS devices is unique in that they operate as mirrors in the static state and as a variable grating in the dynamic state. This unique approach offers significant functional advantages in terms of speed, accuracy, reliability and ease of manufacturing over the common “tilting mirror” MEMS structures. The device was further developed with symmetrical structure, and special membrane and island pattern to achieve polarization-independent and achromatic attenuation as described by Asif Godil, et al. in “Diffractive MEMS technology offers a new platform for optical networks,” http://www.lightconnect.com/technology/Diffractive_MEMS.pdf. The utilization of diffractive MEMS devices for FT spectrometers has not been disclosed in the previous literature.